A conventional thermal fixing device used in an electrophotographic image forming apparatus includes an endless fixing belt that circularly moves, a heater disposed at an internal space defined by an inner peripheral surface of the fixing belt, a backup roller, a heating plate (nip plate) that nips the fixing belt in cooperation with the backup member to provide a nip region, and a guide member for guiding the circular movement of the fixing belt. Specifically, the guide member is configured of an inner guide for guiding the inner peripheral surface of the fixing belt, and an end-face restricting member for restricting a position of the fixing belt (position of an end face of the fixing belt) in an axial direction of the fixing belt. In this fixing device, the end face restricting member is inserted into and assembled to the inner guide in the axial direction after the fixing belt is assembled.